A conventional power supply which is used in an electronic apparatus includes a rectifying circuit and a flattening circuit for converting the supplied power to a DC power of a predetermined value which is suitable for the electronic apparatus. Further, the power supply is constituted such that it is provided with a pulse width modulator for controlling the switching device of the power supply so as to maintain at a predetermined level, and that the duty of the output pulse of the pulse width modulator is adjustable.
FIG. 1 illustrates a circuit of a conventional forward type power supply.
Referring to FIG. 1, the circuit includes a power supply portion 10, a rectifying portion 20 and a filtering portion 30.
The power supply portion 10 is constituted such that: a power supply terminal is connected to one end of a primary coil of a transformer T1 so as to supply a power source voltage Vin; and the other end of the primary coil is connected to a MOS transistor Q1 so that the induction of the power source voltage Vin through the primary coil of the transformer T1 to the secondary coil can be controlled. Further, the gate of the MOS transistor Q1 is connected to a pulse width modulator 12 for outputting pulses to control the turning-on or off of the transistor Q1, while the secondary coil of the transformer T1 coupled with the primary coil, for inducing the power of the primary coil to the secondary coil.
Meanwhile, the rectifying portion 20 is constituted such that the voltage which is induced from the primary coil to the secondary coil of the transformer T1 is rectified through a rectifying diode D1. Further, the voltage is connected with a free wheeling diode D2 for inducing an inverse voltage, when the output from the pulse width modulator 12 is a low signal. The input terminal of the free wheeling diode D2 is grounded.
Meanwhile, the filtering portion 30 includes a smoothing filter which consists of a filter coil L1 and a filter capacitor C1.
Now the operations of the circuit constituted as above will be described. Here, the power source voltage is a DC voltage.
The pulse width modulator 12 supplies high signals and low signals to the gate of the MOS transistor Q1 at predetermined intervals.
When the output signal from the pulse width modulator 12 is high, the MOS transistor Q1 is turned on, so that the voltage Vim supplied through the power supply terminal is induced to the the secondary coil through the primary coil.
The induced voltage is supplied to the rectifying diode D1 to be rectified by it, and then, the rectified voltage is filter by the coil L1 and by the capacitor C1 of the filter portion 30, before being output.
Therefore, a loop is formed through the transformer T1, the rectifying diode D1, the flattening coil L1, the filter capacitor C1 and the transformer T1.
When the pulse width modulator 12 outputs a low signal, the MOS transistor Q1 is turned off, and therefore, the power source voltage Vin which is supplied through the power supply terminal cannot be supplied to the transformer T1, while an inverse voltage is induced in the secondary coil of the transformer T1, and the rectifying diode D1 is turned off. Therefore, the voltage which remains in the coil L1 is flattened by the filter coil, and is output through a power output terminal.
Therefore, a loop is formed through the filter coil L1, the filter capacitor C1, and the free wheeling diodes D2, thereby rectifying the voltage.
In the case where the above circuit is formed, a power loss occurs as much as the diode forward drop voltage due to the existence of the diodes D1 and D2. The diode forward drop voltage refers to the minimum voltage which is required to drive the diode.
Further, the elements are over-heated by the lost power, and in order to prevent the over-heating of the elements, a heat sink is provided. Consequently, the bulk of the circuit is increased, and the manufacturing cost is also increased.
FIG. 2 illustrates another form of conventional rectifying circuits. Referring to FIG. 2, the parts which are same as those of FIG. 1 are omitted as to the description of its operating feature, and the same elements are assigned with the same reference codes.
In order to give solutions to the problems of the circuit of FIG. 1, the rectifying diode D1 and the free wheeling diode D2 of the circuit of FIG. 2 are replaced with MOS transistors Q2 and Q3. As the transistors causing a low power loss, MOS transistors and bipolar transistors are widely used. However, in the case of the bipolar transistors, the switching speed is slower than the MOS transistors, and therefore, they cannot be used in a power supply. Accordingly, transistors are used in a power supply.
Further, the gate of the MOS transistor Q2 is disposed adjacently to the primary coil of the transformer T1, and a tertiary coil is added so as for the power of the primary coil of the transformer T1 to be induced on the tertiary coil, thereby controlling the switching operation of the MOS transistor Q2.
Further the gate of the MOS transistor Q3 is connected to an auxiliary coil T2, so that the coil T2 should be able to control the switching operations of the MOS transistor Q3 by power through the induction from a filter coil L1.
Referring to the above circuit, the operations which are different from that of the circuit of FIG. 1 will be described.
When the output pulse signal of the pulse width modulator 12 is high, the power source voltage Vim which is input through a power supply terminal is induced from the primary coil to the tertiary coil of the transformer T1. This induced voltage turns on the MOS transistor Q2.
Therefore, a loop is formed through the transformer T1, the flattening coil L1, the filter capacitor C1 and the MOS transistor Q2.
When the output pulse signal of the pulse width modulator 12 is low, the switching MOS transistor Q1 is turned off, so that the voltage supplied through the power supply terminal is not supplied to the transformer T1, but that an inverse voltage is induced on the secondary coil of the transformer T1, thereby turning off the MOS transistor Q2.
Under this condition, the energy which remains in the coil L1 is induced to the auxiliary coil T2 which is disposed adjacently to the coil L1. This induced voltage is supplied to the gate of the MOS transistor Q3 to turn on the OS transistor Q3.
Accordingly, there is formed a loop through the filter coil L1, the filter capacitor C1 and the MOS transistor Q3.
However, in the case of the above circuit, the power loss caused by the rectifying diode D1 and the free wheeling diode D2 can be reduced. However, if the voltage which is supplied to the tertiary coil of the transformer T1 becomes higher than the tolerant voltage of the MOS transistor Q2, the MOS transistor Q2 is destroyed.
Therefore, if this problem is to be solved, there has to be provided a device which is capable of blocking the voltage, when the voltage which is supplied to the gate of the MOS transistor Q2 is higher than the tolerant voltage of the MOS transistor Q2. That is, when the induced voltage is higher than the tolerant voltage of the MOS transistor Q2, this voltage has to be blocked by providing an over-voltage blocking portion which consists of a zener diode and a heat releasing device.
In the case where the above circuit is formed, if the equivalent impedance of the output load is high, the energy existing in the filter coil L1 is very small. Consequently, the energy which is supplied to the gate of the MOS transistor Q3 through the auxiliary coil T2 is very small, and therefore, the MOS transistor Q3 cannot perform the switching operations. Further, In the case where the equivalent impedance of the output load is low, the energy which exists on the flattening coil L1 becomes very large, and consequently, the energy which is supplied to the gate of the MOS transistor Q3 through the auxiliary coil T2 becomes very large, with the result that a voltage which is higher than the tolerant voltage of the MOS transistor Q3 is supplied, thereby destroying the MOS transistor Q3.